A liquid cooler including a heat sink. A hollow body is produced by extrusion having at least one flat outer surface between a first and a second end. At least one elongated cavity passes through the body from the first to the second end. The hollow body includes a plurality of parallel fins directed into and along the at least one cavity. An insert housed in the at least one cavity connects the tops of the fins in a fluid-tight manner thereby forming liquid channels for a coolant liquid between the insert and the hollow body. A lid is attached at both ends of the hollow body for forming a liquid cooling system.